Hydrometallurgical Separation Process Of Steel Mill Electric Arc Furnace (Eaf) Dust And The Pigments Obtained By The Process

ABSTRACT

A hydrometallurgical process for the treatment of steel mill electric arc furnace (EAF) dust containing agglomerates of small ferrite particles and larger magnetite particles, the process comprising the steps of: a) washing the EAF dust in water to dissolve soluble salts, metals and simple oxides contained in the dust, said washing step being performed under agitation and with an alkaline pH; b) decanting the solution of step a) to obtain a supernatant liquid containing the dissolve salts, metals and simple oxides and a slurry containing ferrites and magnetites, a non toxic amount of leachable lead and a reduced amount of calcium; c) separating the slurry and the supernatant liquid; d) adding to the slurry obtained in step c) an anionic surfactant to disperse the ferrite particles adsorbed on the magnetite particles; and e) treating the slurry from step d) to produce pigments selected from the group consisting of ferrite pigments, magnetite pigments and ferrite/magnetite pigments.

FIELD OF THE INVENTION

The present invention relates generally to the field of steel mill dusttreatment. More particularly, the invention comprises ahydrometallurgical separation process of dust produced by electric arcfurnaces in steel mills. This process permits, on one hand, thedecontamination of the dust and on the other hand, the production offerrite and/or magnetite pigments useful in paints, plastics andconcrete. The invention also comprises the pigments produced from thisprocess.

PRIOR ART

Electric arc furnace (EAF) dust, also known under the name of (K061), isclassified as a dangerous material because it contains highconcentrations of soluble heavy metals such as cadmium, zinc, chromiumand lead, but in particular lead. More specifically, EAF dust usuallycontains more than 5 ppm soluble lead and hence, does not meet thelimits of lead specified by TCLP (Toxicity Characteristic LeadingProdecure). This dust also contains spinel compounds, notably magnetite(Fe₃O₄) and diverse ferrites (MO₂Fe₂O₃). These spinel compounds as wellas contaminants appear in the form of agglomerates and aggregates. Tothe naked eye, the dust is brown and an observer, even with the aid of amagnifying glass, will not notice the presence of black balls ofmagnetite, even if certain black balls can attain 150 μm in diameter.The brown ferrite contained in the dust is ultrafine, and as a pigment,coats by adsorption the larger particles of magnetite.

Table 1 shows the typical chemical composition of EAF dust coming fromtwo distinct steel mills. These compositions show elevatedconcentrations of certain heavy metals. TABLE 1 CHEMICAL ANALYSES OF EAFDUST COMING FROM TWO DISTING STEEL MILLS IN THE PROVINCE OF QUEBECSAMPLES Elements Code Units MILL 1 MILL 2 AL ICP90 ppm 7100 4500 BaICP90 ppm 157 120 Ca ICP90 ppm 107800 146000 Cd ICP90 ppm 153 200 CoICP90 ppm 14 61 Cr ICP90 ppm 1200 1400 Cu ICP90 ppm 1720 1700 Fe ICP95ppm >30 258000 K ICP90 ppm 17700 7400 Mg ICP90 ppm 49200 22200 Mn ICP90ppm 15300 27200 Mo ICP90 ppm 18 41 Na ICP95 ppm 33300 9700 Ni ICP90 ppm125 130 P ICP90 ppm 500 670 Pb ICP90 ppm 10950 9500 Si ICP95 ppm 1550015800 Ti ICP90 ppm 700 600 V ICP90 ppm 98 n.d. Zn ICP90 ppm 93900 162000

Most EAF dust treatment processes in the prior art aim at recovering orremoving the heavy metals in an “aggressive” manner, attacking thecristallographic structure of the spinels.

Also known in the prior art, is EP 0 853 648 (equivalent to U.S. Pat.No. 6,022,406), which describes a hydrometallurgical process of EAF dusttreatment with the aim to produce pigments. This process comprises astep of magnetic separation of the dust into two fractions, one fractioncontaining less magnetic elements, and the other fraction containing nonmagnetic elements, as well as treatment steps of these two fractions toobtain zinc ferrite pigments. The process disclosed also has as aneffect to attack the cristallographic structure of spinels other thanthe zinc ferrite spinel, and in this sense, is also an aggressiveprocess.

Therefore, there is presently a need for a treatment process of EAF dustthat permits an efficient and unagressive recuperation of the differentferrites and magnetites present in the dust, as well as permitting thedecontamination of the dust.

SUMMARY OF THE INVENTION

One objective of the invention is to propose a treatment process of EAFdust that responds to this need.

According to the present invention, that objective is accomplished witha hydrometallurgical process for the treatment of steel mill electricarc furnace (EAF) dust containing agglomerates of small ferriteparticles and larger magnetite particles, the ferrite particles coatingby adsorption the larger magnetite particles, the dust furthercontaining calcium oxide, zinc oxide and a toxic amount of leachablelead together with minor elements selected from the group consisting ofMg, Cr, Cu, Cd, V, and chlorides. The process comprises the steps of:

a) washing the EAF dust in water to dissolve soluble salts, metals andsimple oxides contained in the dust, the washing step being performedwith an alkaline pH;

b) decanting the solution of step a) to obtain a supernatant liquidcontaining the dissolved salts, metals and simple oxides, and a slurrycontaining ferrites and magnetites, a non toxic amount of leachable leadand a reduced amount of calcium;

c) separating the slurry and the supernatant liquid;

d) adding to the slurry obtained in step c) an anionic surfactant todisperse the ferrite particles adsorbed on the magnetite particles; and

e) treating the slurry from step d) to produce pigments selected fromthe group consisting of ferrite pigments, magnetite pigments andferrite/magnetite pigments.

Preferably, the sequence of steps a) to c) is performed more than onetime before adding the anionic surfactant.

The use of an anionic surfactant was found to increase the efficiencyand quality of further separation steps such as screening, andferrite/magnetite separation by a magnetic separator. Steps a) to c)also enable the decontamination of the dust by leaching salts, metalsand simple oxides such as lead oxide. This selective solubilization isdue to the alcaline pH solution, which is preferably greater than 12,resulting from the first washing, and optional second washing, withwater. This alcalinity promotes the solubilization of PbO and, with theaddition of surfactant, enables the product to pass the test set out bythe TCLP, which regulates standards of dangerous materials.

Advantageously, the process of the invention also enables the separationof the ferrites from the magnetites without breaking thecristallographic structure of the spinels, so as to produce magnetiteand/or ferrite pigments of different grades, whose differentcompositions have commercial values.

The process also permits the decontamination of EAF dust byhydrometallurgical means while maintaining the most stable families ofspinels intact.

The solution obtained is step a) described above has a positive zetapotential, and the anionic surfactant is preferably added in aconcentration sufficient to reduce the zeta potential to or close to theisoelectric point, and more preferably to the isoelectric point.

The anionic surfactant is preferably a phosphate or an equivalentthereof. More preferably, sodium metaphosphate is used as thesurfactant.

The use of sodium metaphosphate presents the following additionaladvantages to the process. Sodium metaphosphate converts the calcium andcalcium hydroxydes present in the liquid phase into a calcium phosphatewhich is precipitated with the solid. Therefore, this form of calciumsequestering allows for a quicker and sharper fractionation of theslurry by, for example, a drum magnet, and in addition when the slurryis eventually separated by screening, clogging of the mesh opening isminimized, and therefore requires less cleaning.

Step e) of treating the slurry preferably comprises the step ofmagnetically separating the slurry into a first fraction composedessentially of brownish ferrites and a second fraction composedessentially of black magnetite, the first fraction being less magneticthan the second fraction.

The magnetic separation is preferably performed with a magnetic field inthe range of 400 to 700 gauss, more preferably around 550 gauss.

In accordance with preferred aspects of the invention, the processfurther comprises steps of treating the first fraction to produceferrite pigments and/or treating the second fraction to producemagnetite pigments.

Treatment of the First Fraction (Ferrite)

The step of treating the first fraction preferably comprises the stepsof:

-   -   removing from the first fraction, particles having a grain size        of 20 μm or more, to obtain a refined first fraction;    -   leaching the refined first fraction with a solvent, to obtain a        leached slurry;    -   separating the leached slurry into a solid fraction containing        ferrite pigments and a liquid fraction containing constituents        of the first fraction soluble in the solvent; and    -   drying the solid fraction to obtain dry pigments of ferrites.

In accordance with a first variant, the solvent is water and the ferritepigments obtained are ferrite pigments of a first grade.

In accordance with a second variant, the solvent is sulphuric acid, theleaching is performed at a pH of 0.5 to 3 and the ferrite pigmentsobtained are ferrite pigments of a second grade.

In accordance with a third variant, the solvent is nitric acid, theleaching is performed at a pH of up to 3, and the ferrite pigmentsobtained are ferrite pigments of a third grade.

In accordance with a fourth variant, the process further comprises thestep of wet grinding the solid fraction to obtain a fourth grade ofpigments having a finer mean grain size and a lower concentration oflead as compared to the ferrite of the third grade.

Treatment of the Second Fraction (Magnetite)

The step of treating the second fraction preferably comprises the stepof screening at 6 μm to obtain a first finer fraction with particleshaving a grain size of 6 μm or less, and a coarser fraction withparticles having a grain size greater than 6 μm.

In that case, the process preferably further comprises the steps of:milling the coarser fraction, and removing from the milled coarserfraction the particles having a grain size greater than 40 μm andreturning these particles for further milling, and a second finerfraction having particles with a grain size of less than 6 μm, resultingin the coarser fraction containing particles having a grain size between40 and 6 μm.

In accordance with a fifth variant of the process, the coarser fractionis preferably wet grinded by attrition to attain a mean grain size ofapproximately 0.3 μm.

The grinded product is thereafter filtered and dried to obtain amagnetite pigment of a first grade.

In accordance with a sixth variant, the first and second finerfractions, which contain particles of less than 6 μm, are purified bysuspending residual contaminants contained therein with an anionicsurfactant, to obtain a purified magnetic fraction. The purifiedfraction is thereafter decanted, wet grinded by attrition, filtered anddried, to obtain a magnetite pigment of a second grade.

Production of Magnetite/Ferrite Particles

In accordance with a seventh variant of the invention, which does notinvolve magnetic separation, the process preferably comprises the stepsof:

-   -   removing from the slurry obtained in step d), particles having a        grain size of 60 μm or less, to obtain a refined slurry;    -   leaching the refined slurry in nitric acid at a pH of about 3,        to obtain a leached slurry with no or a controlled amount of        ZnO, which retards the setting of concrete;    -   separating the leached slurry into a solid fraction containing a        mixture of ferrite and magnetite pigments and a liquid fraction        containing constituents soluble in nitric acid; and    -   drying the solid fraction to obtain dry pigments containing a        mixture of ferrite and magnetite.

The pigments obtained with this variant are suitable for use in concreteformulation for retarding the setting of concrete or for coloring thesame.

All the seven variants described above also preferably comprise thesteps of:

-   -   coating the pigments with an inorganic and/or organic coating;        and    -   micronizing the coated pigments.

The present invention also concerns ferrite pigments and/or magnetitepigments or a mixture thereof, obtained by the processes describedabove. It also concerns a ferrite pigment from EAF dust, showing aresistance to leaching; and preferably showing a color thermal stabilityat temperatures of 300° C. and higher.

Preferably, the ferrite pigment provides anticorrosion properties tometallic paint formulation.

The present invention also concerns the use of a ferrite pigment asdescribed above for incorporation in anticorrosive paint formulation,plastic formulation or concrete formulation; and the use of a magnetitepigment as described above for incorporation in a paint formulation,plastic formulation or toner formulation to provide magnetic properties.

BRIEF DESCRIPTION OF THE DRAWING

The characteristics of the present invention will be best understood byreading in a broad manner the following description of a preferredembodiment for carrying out the invention, while referring to theannexed drawing in which:

FIG. 1 is a flow chart of the process according to a first variantsuitable for producing ferrite pigments of the first grade.

FIG. 2 is a flow chart of the process according to a second variantsuitable for producing ferrite pigments of the second grade.

FIG. 3 is a flow chart of the process according to a third variantsuitable for producing ferrite pigments of the third grade.

FIG. 4 is a flow chart of the process according to a fourth variantsuitable for producing ferrite pigments of the fourth grade.

FIG. 5 is a flow chart of the process showing a fifth and a sixthvariant suitable for producing magnetite pigments of the first grade andthe second grade.

FIG. 6 is a flow chart of the process according to a seventh variantsuitable for producing ferrite/magnetite pigments.

FIG. 7 is a graph showing extraction values for calcium, chromium, zincand lead versus time, and using a hydrofoil impeller.

FIG. 8 is a series of graphs representing the variation of the zetapotential, ph, and conductivity versus the concentration of sodiummetaphosphate for a partly washed dust slurry.

FIG. 9 is a graph showing extraction values for calcium, chromium, zincand lead versus time, and using a high shear impeller.

FIGS. 10 to 13 are graphs showing the granulometric distribution of thefirst fraction (ferrite fraction) after one or two passes in the grinder

FIG. 14 is a photo of ferrite pigments taken with an AFM microscopeafter wet grinding by attrition, showing the state of agglomeration andthe fine size of the constituent ferrite fragments.

DESCRIPTION OF PREFERRED PIGMENTS PRODUCED WITH THE PROCESS OF THEINVENTION

Four grades of ferrite pigments, two grades of magnetite pigments andone grade of ferrite/magnetite pigments were produced in the pilot run.The ferrite pigments were produced according to the first, second, thirdand fourth variants of the process shown in FIGS. 1 to 4; the magnetitepigments were produced according to the fifth and sixth variant of theprocess shown in FIG. 5, and the ferrite/magnetite pigments wereproduced according to the seventh variant of the process shown in FIG.6.

All pigment grades have been obtained by a substantially similartreatment. The processes use the same physical manipulation techniques,but differ in the specific leaching step which gives the pigments theirdesired chemical and surface characteristics. In many cases, specificcoating also gives the pigments even greater specific properties formore particular markets.

The novelty of the process for all grades of pigment resides in aninitial treatment of the EAF dust with water with the addition of ananionic surfactant. This surfactant increases the efficiency and qualityof the ferrite/magnetite separation by the magnetic separator. Thisinitial treatment also enables the decontamination of the dust byleaching salts, metals and simple oxides such as lead oxide. Thisselective solubilization is due to the alcaline pH>12 solution resultingfrom the first washing (first mixing) and rinsing (second mixing) withwater (Table 2). This alcalinity promotes the solubilization of PbO andenables for the product to pass the test set out by the TCLP, whichregulates standards of dangerous materials (Table 3). TABLE 2 WATERANALYSES First Treatment Chemical Analyses Zn Pb Cr Cd Ca ppm ppm ppmppm ppm S99# 28 1T H2O 1.36 170 0.75 0 770 S99# 28 1T H2O/LV 1.56 410.95 0 340 B99# 79 1T H2O 3.21 71.7 15.74 0 206 B99# 79 1T H2O/LV 2.0065.4 1.93 0 492

TABLE 3 TCLP RESULTS Analysis of leachate FM Pb Cr Cd Zn # Grade pH ppmppm ppm ppm 1076 Ferrite pigments of 8.5 0 0.32 0 0 the first grade 1084Ferrite pigments of 8.7 0 0 0 0.17 the first grade 1104 Ferrite pigmentsof 2 0.20 2 240 the first grade

Ferrite Pigments

First Grade (F1)

The ferrite pigment of the first grade was produced with the aid of asolution containing an optimal concentration of surfactant, theconcentration being a function of the isoelectric point of the dust tobe treated, and with a leaching hereafter referred as to the secondtreatment) with water only.

The first grade ferrite pigment contained a high quantity of lead thatcannot be easily leached under normal pH conditions. After ten monthsand many agitations in water, this pigment showed no leaching of heavymetals (Table 4) and is comparable to pigments of the second and thirdgrade described below. Heavy metals, with the exception of 8% zinc inthe resistant form of zincite, wave present and stabilized in thestructure of certain ferrites and spinels. TABLE 4 WATER LEACHATEANALYSES OF 10 MONTHS FM# 1061 1082 1084 1133 1076 Grade B-HS4 B-HSThird Second B-PE B-PE S-PE grade grade First grade First grade Firstgrade Chemical Analysis Pb ppm 2.0 UDL UDL 1.0 UDL Cd ppm 0.1 0.3 UDLUDL 0.10 Cr ppm 0.14 UDL UDL 0.01 0.53 Zn ppm 3.53 0.06 0.1 UDL 0.04 Cappm 9.1 18.8 24.6 90.2 107.0 Fe ppm 15.01 0.14 UDL UDL 0.19Sedimentation 1.6 2.2 2.4 2.5 3.5 on (cm) pH 7.60 7.79 12.03 12.41 12.45

The varied acid leaching steps of the process left solid ferrites ofvaried compositions and, as experience has taught, the ferrites rich inCa were less stable to leachings than zinc ferrites or other ferritesrepresenting complex oxides of Ca, Fe, Zn, Mn, Mg, Ni, Cr, etc. Theresistant ferrites left after leaching, which made up the pigment, gavethe pigment a high thermal stability and resistance to leaching, whichare a function of the ionic stoichiometry and of the type and quality ofthe composite cristalline structures.

On the other hand, the ferrite pigments of the first grade demonstratedhigh resistance to corrosion as demonstrated in the salt spray (mist)tests, allowing coated metallic plates to resist corrosion for more than1500 hours in a salt mist, which is equal or superior to all otherpigments, including those of commercial quality used in the tests.

The first grade ferrite pigment owes its corrosion (salt mist)resistance to CaO, which is sacrificed as Ca(OH)₂ and/or to theresulting alcaline viscosity (soapy appearance) associated with Ca(OH)₂and the pigment's elevated alcalinity.

Second Grade (F2)

The ferrite pigment of the second grade was produced in the same way asthe first grade, except that the second treatment was performed withsulphuric acid.

The preparation steps for the second grade pigments were identical tothose used for the ferrite pigments of the first grade, the addition ofthe surfactant occurring after the first washing but before the magneticseparation. For the second grade pigments, leaching using sulphuric acidat a pH between 0.5 and 3 allowed for the preservation of a certainquantity of hydrated calcium sulphate, the solubilization of all the Znin the form of zincite (ZnO) and the stabilization of lead as a solidsulphate. Using this treatment, the effluents rich in zinc sulphate, area suitable form of compound to be directly recycled back into anelectrolysis process, in order to recuperate the value of the zinc. Thecalcium sulphates generated by the leaching are not harmful inanticorrosion paints. Calcium sulphate is frequently used as a fillerwith pigments used in paints and is often desirable as a pigmentaryadditive. This pigment did not require wet grinding by attrition, nordid it require a second magnetic separation and a second screening afteracid leaching. The pigment was filtered in order to obtain an allowablesoluble salt concentration of 0.3 g/l mg, and was then dried andmicronized.

In consequence, the second grade of ferrite pigments allowed for theconservation of a fraction of calcium, the transformation of lead oxideinto lead sulphate (which is very stable) and the solubilization of zincoxide into zinc sulphate. These characteristics of the second gradepigment make this pigment an excellent colorant as well as a corrosionresistant pigment.

Third Grade (F3)

The ferrite pigment of the third grade was produced in the same way asthe first grade, except that the second treatment was performed withnitric acid.

The leaching with nitric acid enabled the preferential removal of leadand other heavy metals due to the oxidizing property of the acid. Theleaching was performed at a pH between 0 and 3, which permitted theelimination of certain families of ferrites, as a function of the pH, inorder to minimize the total lead in the pigment and to give a pigmentwith a particular signature with regards to its composition, structureand surface characteristics. As an example, between a pH of 3 and 1.5,the ferrites displayed a zeta surface potential that is positive, butthis potential became negative at a pH<1.5. This charge characteristicinfluenced the acceptable coatings and their associated mechanisms.

The different ferrites issuing from these Teachings, showed high heatresistant capabilities, which are very valued pigmentary properties.This leaching also minimized the difference between the pigment coloursand enabled a delta of variation of about 0.5 for pigments of differentdust deliveries (Table 5). The properties were equal or superior to thepigments currently recognized on the industrial market. This grade ofpigment showed enhanced resistance to corrosion depending on the coatingused and also displayed a thermal stability as they preserve theircolour tint at temperatures exceeding 300 to 400° C. TABLE 5 E VARIATIONOF FERRITE HS4 PIGMENTS (THIRD GRADE) FROM MILL 1 COLOR FM # L a b DeltaE 1317 27.13 2.27 7.38 0.2 1318 27.1 2.36 7.35 0.15 1319 26.81 2.34 7.150.21 1320 26.82 2.4 7.25 0.17

This thermal resistance is a requirement for plastics, powdered paintand ceramics.

Fourth Grade (F4)

The ferrite pigment of the fourth grade was produced in the same way asthe third grade, with the addition of a wet grinding step.

This pigment can be used in concrete as a cement additive that increasesthe fluidity and compression resistance of the concrete. This pigmenthad a finer granulometry than the third grade, ferrite pigment and theferrite/magnetite pigment.

The ferrite pigments of the first, second, third and fourth grades haveapplications in anticorrosive paints. The third grade can be used inplastics and powder paints due to its thermal resistance. This pigmentcan also be used as a cement additive, thinning agent and additive inhigh performance concrete. The major difference between the second andthird grade ferrite pigments lies in their surface properties.

Magnetite Pigments

First Grade (M1)

The magnetite pigment of the first grade was produced by grinding with aball mill the magnetic fraction issuing from the magnetic separation.The ground fraction was passed through a screening between 38 and 6 μm,and wet grinding by attrition in order to result in a mediangranulometry of about 0.3 μm. The pigment was then filtered, coated withan organic coating, dried and micronized.

Second Grade (M2)

The magnetite pigment of the second grade was obtained by screening themagnetic fraction, which had already undergone ball mill grinding, at 6μm. This fraction was purified by putting the silica, carbonate andresidual ferrite contaminants into suspension, with the aid of ananionic dispersive surface active.

More particularly, this pigmentary grade of magnetite was obtained byscreening at 6 μm the magnetic fraction of the magnetic separation andthe fractions less than 6 μm coming from the screening of the roughmagnetite after its ball mill grinding. This fraction, which contained aconcentration of magnetite, was purified by putting the silica carbonateand ferrite residue contaminants into suspension with the help of asurfactant. Two successive treatments of adding surfactant, followed bya decantation of the magnetite and separation of the suspension, wererequired to obtain an adequately black product which was subjected towet grinding by attrition in order to attain a desired granulometry. Thesolid was finally filtered with an organic additive, dried andmicronized. This step of purification is similar to the first treatmentof the dust. The ferrites and contaminants were put into suspension, andthe magnetite was decanted.

A rough non pigmentary magnetite was also produced. It was obtainedafter attrition grinding the magnetic fraction coarser than 30 μm. Theattrition cleans the surface of the magnetite spheres by wearing out thewhite coating of calcium and silicate initially present. This stepimproves the black color of the spheres and eliminates the magnetiteswhich are less resistant to abrasion. The 70 and 30 μm product can beused as a toner in photocopy processing. The commercial niche of thissolid depends on its granulometry, morphology, resistance to frictionand magnetic properties.

Ferrite/Magnetite Pigments (FM)

Ferrite/magnetite pigment suitable as a colorant for concrete wasproduced with nitric acid at a pH of 3 but without magnetic separation.The slurry from the first treatment was subjected to the followingsteps: screening at 6 μm, leaching in nitric acid, filtration in orderto reduce its soluble salt content, and drying in a flash dryer,yielding a coarse pigment made up of agglomerates having a median grainsize of 5 μm.

The screening enabled the removal of coarser contaminants includingsilica, coal and other fragments. After this, the slurry containing amagnetic charge underwent leaching with nitric acid at a pH of 3 inorder to remove the zincite, since zincite delays the setting of cement.The product was filtered in order to reduce its soluble salt content,after which drying in a flash dryer gave the pigment a granulometry witha median of about 5 μm.

For this grade of pigments, the initial pilot process was greatlysimplified, which translates into a reduced production cost.

Because its granulometry is too large, the pigment cannot be used as anadditive in cement, in order to make high performance concrete.

For all grades of pigments, whether ferrite pigments, magnetite pigmentsor ferrite/magnetite pigments, at the end of the treatment, an organicadditive was provided for the finished product in order to standardizethe surface charges, to facilitate the incorporation of the dry pigmentinto paint resins, and to give a desired fluidity for its handling. Itis however worth mentioning that the coating step is optional to theprocess.

DESCRIPTION OF PREFERRED VARIANTS OF THE PROCESS ACCORDING TO THEINVENTION

The process for treating EAF dust according to the invention is ahydrometallurgical process for the treatment of steel mill electric arcfurnace (EAF) dust that contains agglomerates of small ferrite particlesand larger magnetite particles, the ferrite particles coating byadsorption the larger magnetite particles, the dust further containingcalcium and toxic amount of leachable lead together with minor elementsselected from the group consisting of Mg, Cr, Cu, Cd, V, and chlorides.

Ferrites represent a complex family of compounds represented chemicallyby the major elements Ca, Fe, Zn, Mg, which are the major and importantelements in this process together with minor elements selected from thegroup consisting of manganese, chromium, copper, cadmium, lead, vanadiumand chlorides. Most of the elements are represented as oxides; eithercomplex oxides like the ferrites or simple oxides represented by PbO,ZnO, CaO some other salts and metals are also present. This process alsoapplies to EAF dust with low zinc content generated from the use ofpre-reduced iron ore pellets of hematite.

The process steps according to different preferred variants of theprocess are illustrated in FIGS. 1 to 6, for the different grades ofpigments. They show a hydrometallurgical batch process with noatmospheric emissions. The dust slurry of the first washing step iscomposed essentially of ferrites (65-75%), magnetites (20-28%), zincite(ZnO) and litharge (PbO) (8%), CaO/Ca(OH)2 (5-12%) and variableconcentrations of silica and coal.

One difference between the process of the invention and the prior art ofEAF dust treatment processes, lies in the fact that the profitability ofthe present process is not a function of the zinc concentration of theEAF dust. One of the steel mills that will be seen in an example uses anEAF feed of at least 50% pre-reduced hematite, with 50% scrap iron ofdifferent grades. Depending on the required production, the percentageof hematite and scrap iron can vary. For this steel mill, the dust'saverage zinc concentration is close to 9% compared to 16-22% for dustgenerated from feeds composed of scrap iron only. Table 1 shows twochemical analyses of EAF dust from the two steel mills in Quebec, thatwere used for testing the process.

The optimization and characterization of the test pilot run wereeffected by:

-   -   conducting physiochemical analyses: chemical analyses,        granulometric distribution tests, and identification of chemical        phases by X-ray diffraction and electronic microscopy, etc.;    -   optimizing the efficiency of the yield at different stations by        measuring the volume and concentration (g/l) of the slurry, the        weights of their solid fractions, and the processing time; the        pH and electric conductivity of the liquids being also measured,        etc.;    -   noting the pH and the electrical conductivity of the liquids;    -   evaluating the pigments by noting the colour specifications of        the solid, humidity, oil absorption capacity, quality of        dispersion, and salt mist tests, etc.

First Treatment

The process comprises a first treatment which essentially consists ofwashing and rinsing the EAF dust for reducing the amount of calcium andsoluble lead, to thereafter facilitate further treatment of the dust toproduce commercial grade pigments.

More specifically, the first treatment, which is performed in a tank(10) comprises the steps of:

a) washing the EAF dust (12) in water to dissolve soluble salts, metalsand simple oxides contained in the dust, the washing step beingperformed with an alkaline pH which is preferably greater than 12;

b) decanting the solution of step a) to obtain a supernatant liquid (14)containing the dissolve salts, metals and simple oxides and a slurry(16) containing ferrites and magnetites, a non toxic amount of leachablelead and a reduced amount of calcium;

c) separating the slurry (16) and the supernatant liquid (14); and

d) adding to the slurry obtained in step c) an anionic surfactant (18),preferably a phosphate and most preferably sodium metaphosphate, todisperse the ferrite particles adsorbed on the magnetite particles. Itis worth mentioning that another anionic surfactant known in the art andthat would have the same effect of dispersing the adsorbed ferriteparticles is within the scope of the present invention.

Preferably, the sequence of steps a) to c) is performed more than onetime before adding the anionic surfactant. Note that steps a) to c) arenot shown in the figures.

Steps a) to d) are performed in the tank (10) shown in each of FIGS. 1to 6. After the first treatment, the slurry (16) from step d) is sent tofurther stages of the process to produce pigments selected from thegroup consisting of ferrite pigments, magnetite pigments andferrite/magnetite pigments.

The treatment of the slurry (16) will vary depending on the grade offerrite or magnetite to be produced. The production of each of thesegrades according to the process of the invention will be described infurther detail further below.

EXAMPLE

An example of the first treatment is now described in further detail.

Washing and First Agitation

The EAF dust was washed with water under agitation provided by ahydrofoil impeller with a rotation speed of approximately 350 rpm in atank. The height of the fluid level and the tank diameter had a ratio of1:1. The tank agitation system also comprised four baffles, which actedas static agitators.

The concentration of the slurry was 16%. Tests were made with batches of10, 20 and 30 kg of dust for 60 liters of liquid, corresponding to solidconcentrations of 16, 32 and 48% respectively.

The washing provided:

-   -   an aqueous solution of alkaline pH>12;    -   a dissolution of soluble salts, heavy metals and simple oxides        under alkaline conditions (see Table 2) (this chemical charge        was the liquid that is eliminated by decanting and pumping the        supernatant liquid);    -   the initiation of the break-up of ferrite particles that are        weakly linked (FIG. 7);    -   the dissolution of CaO and some calcium ferrites into soluble        calcium and CaOH₂, and the dissociation of lead oxide;    -   the transformation of the CaO fraction abundant in CaOH₂ and the        dissolution of lead oxide;    -   It is worth mentioning that greater agitation with other types        of cutting agitators may unfavorably result in elevated        concentrations in Ca and Pb in the liquid, which could hinder        subsequent treatment steps.    -   The duration of agitation was 60 minutes and it was followed by        a decantation period of 60 minutes and a separation of the        supernatant liquid.

Given the high specific weights of the ferrites and magnetites,decantation of the slurry solid was used instead of filtration.

Rinsing and Second Agitation

The slurry from the washing was rinsed with water. Water is preferable:

-   -   to recuperate the metals and alcaline water of pH 12 from the        interstitial water in the 20 liters of residual pulp of the        first mixing; and    -   to continue the leaching of the calcium, lead and zinc in the        dust.

The rinsing was carried out for a period of 60 minutes, followed by a 60minute decantation and recuperation of the supernatant liquid.

Addition of Surfactant

The addition of a surfactant had various objectives in the process.Firstly, it reduced the positive charge of the fine particles of thepulp represented by a zeta of 32 mV in order to attain the isoelectricpoint (zeta of 0 mV) for the system (slurry). This reduction of thecharge of the chemical phases of the system facilitated thefractionation of the composites. Further details on the effect of thesurfactant on the charge of the chemical phases are given in the sectionentitled “Magnetic separation” hereinbelow. Secondly, when a phosphatesuch as sodium metaphosphate was used, the surfactant temporarilyconfined the CaO coming from the ferrites by coating the surface of theparticles with phosphate. Also, the surfactant was able to convert thecalcium already in solution into calcium phosphate, which was insolublein the solution and was concentrated with solid. It was also believedthat some of the lead in solution is also precipitated in the form of alead phosphate or in the form of a calcium and lead phosphate phase.

These deductions are supported by titration of the slurry with sodiummetaphosphate, which is the preferred surfactant (FIG. 8). Theconditions of the tests whose results are shown in FIG. 8 were: 5% ofsolid in a slurry of 240 ml; titration with the sodium metaphosphate of3.6% (w/w) (22 ml of the solution was used); zeta potential calculatedby using a laser volume median; S.G. 4 g/cc. The graph showing Zeta vs.Surfactant Concentration, shows the reduction of the positive chargedown to the isoelectric point. The graph showing Conductivity vs.Surfactant Concentration represents the concentration of ions in thesupernatant liquid, which decreases with the addition of surfactant.

After the addition of surfactant, agitation was resumed in order tostandardize the state of the mixture and the feed of the magneticseparator which was fed at a flow rate of 1 l/min. The slurry was fedinto the magnetic separator while agitating in such a way as to maintainthe slurry homogenous in its magnetite and ferrite content throughoutthe tank.

After the agitation step and the two decantation steps, the alkalinesolutions of the effluents (80 liters) were used in the effluenttreatment. The alkaline liquid was mixed with the acid effluents of thesecond treatment which will be described further below, in order toneutralize their acidity and to promote precipitation of the metals insolution.

This first treatment (washing) of the raw EAF dust, which generated analkaline solution, also promoted the solubilization of soluble salts insimple lead and zinc oxides to a concentration that satisfies thegoverning standards of the TCLP test, and the rules governing dangerousmaterials. In other words, the leachate of the dust did not exceed theTCLP (Table 3) standard and thus is neither considered as contaminated,nor held under the rules of dangerous materials.

The Role of Agitation in the First Treatment

Agitation tests were performed under variable times from 15 to 60minutes, using a hydrofoil impeller. The resulting granulometry of thesolid fraction of the slurry, was obtained by a granulometer able tomeasure to the scale of a nanometer, according to the settings ofnumber, surface and volume (FIG. 7). The corresponding granulometricvariation after 60 minutes indicated an acceptable size, with a medianof roughly 0.6 μm for the solid. This diameter was further reducedduring the leaching of the second treatment. With other impellers, whichhad higher shear levels, the resulting granulometry was too fine foroptimizing the first treatment. Other agitation tests, in which thesurpernatant liquid was analyzed after filtration for lead and calciumcontent, were performed and the results are presented in Table 6, and inFIG. 9. The chemical concentrations resulting from the test with thehydrofoil impeller indicated a stable concentration for an agitationtime of 60 min. This agitation time represented a maximum for theextraction of calcium, and a plateau of saturation for the value oflead. For the other impellers, the elemental concentrations were toohigh, and thus not optimized. TABLE 6 LEACHING OF EAF DUST DURING THEFIRST TREATMENT AT VARYING TIMES Run#1: 10 kg on the system of 1T of thepilot plant; Results of the AAS Analysis time Pb Ca Cr Zn min. ppm ppmppm ppm pH  0 22 0.6 39.71 4.88 15 60 111.1 35.53 9.72 30 72 166.1 33.36.48 45 78 180 33.3 6 60 78 190.3 27.17 9.18 Washing 15 77 323.4 5.313.7 12.67 30 77 352 4.92 3.74 12.69 45 88 226.6 4.5 4.18 12.71 60 88172.7 4.81 4.36 12.72

After the first treatment which is performed in the tank (10) of FIGS. 1to 6, the slurry (16) is either sent to the magnetic separation (20) toseparate the magnetite particles and the ferrite particles, as in FIGS.1 to 5, which show the first to sixth variants; or it is sent toscreening (30) and thereafter to the second treatment (40), as in FIG. 6which shows the seventh variant, to ultimately produce a pigment offerrite and magnetite suitable for use as a colorant for concrete.

The first to the sixth variants, which concern the production of ferritepigments (FIGS. 1 to 4) and the production of magnetite pigments (FIG.5), will now be described in further detail while referring to FIGS. 1to 5. For each of these variants, as broadly described, the slurry (16)from the first treatment was subjected to a magnetic separation (20) toobtain a ferrite fraction (24) and a magnetite fraction (26). Both thesefractions (24 and 26) were respectively subjected to a screening (30 or32).

Referring to FIGS. 1 to 4, the refined ferrite fraction (34) from thescreener (30) was further subjected to a second treatment (40) dependingon the grade of ferrite pigments produced. In the case of the third andfourth variants (FIGS. 3 and 4), the second treatment was preceded by atleast one of the following steps: decantation (60), grinding (50 or 55),and magnetic separation (200). After the second treatment (40), theslurry (46) obtained was subjected to filtration (70), and thereafter tothe typical process steps used in the field of pigment production, asfor example drying (90), coating (80) and micronization (100).

The filtration step (70) produces water to be recycled (72).

It is also worth mentioning that in the first and the third processvariants, the second treatment was preferably followed by a secondmagnetic separation (200, 220) used to separate the magnetite fraction(202, 222) that remained in the slurry (46) from the ferrite fraction(206, 226). The ferrite fraction (206, 226) was sent back to the ferriteproduction line for producing the ferrite pigments, whereas themagnetite fraction (202, 222) was sent to the magnetite production line.

Referring to FIG. 5, the magnetic fraction (26) from the magneticseparation (20) was sent, preferably with magnetic particles (202, 212,222) from other steps of the process, to a first screening (30) at 150μm. The fraction (38) of less than 150 μm was sent to a ball mill (500)and then to a second screening (32) to obtain a first finer fraction(304) with particles having a grain size of 6 μm or less; and a coarserfraction (306) with particles having a grain size greater than 6 μm. Thecoarser fraction (306) was then milled and screened at 40 μm (thesesteps are not shown on FIG. 5) to finally obtain a coarser fractioncontaining particles having a grain size between 40 and 6 μm.

The coarser fraction (306) was wet grinded by attrition (50) to attain amean grain size of approximately 0.3 μm. It was thereafter subjected tothe typical process steps used in the field of pigment production, asfor example drying (90), coating (80) and micronization (100).

The finer fractions (304) were purified by suspending (600) residualcontaminants contained therein with an anionic surfactant (802), toobtain a purified magnetic fraction (602).

Magnetic Separation (20)

The magnetic separation step (20) yields the first fraction (24)containing in a major portion ferrite particles and the second fraction(26) containing in a major portion magnetite particles.

In the raw EAF dust, the black magnetite is never apparent or visible tothe naked eye, even though the magnetite is large and rough compared tothe other components of the dust. This phenomenon is explained by theadsorption of the ferrites to the surface of the magnetites. In the rawdust, the ferrites are positive and the magnetite is negative, whichcauses an electrostatic attraction between these two chemical phases.This charge can be measured with an apparatus called “ElectroacousticSonic Amplitude (ESA)”, which enables the calculation of the zetapotential of the particles in aqueous medium, and the indirect andqualitative evaluation of the surface charge of the particles. Theresults indicate that ferrites have a positive charge with a zeta of +27mV, whereas the magnetites are lightly negative and have a zeta of −3mV, which corresponds to the charge values for naturally occurringmagnetites. Also, given that the ferrites have a granulometry under 1μm, they will coat the large rough surface of the magnetite. This roughtexture of the magnetite surfaces seems to be produced by the depositionof phases of calcium and other composites which can be removed byattrition. These factors render it difficult to separate ferrites frommagnetites. Laboratory experience teaches us that without a surfactant,it is possible to obtain a fraction concentrated in magnetite, but thisfraction is brown and not black, and has a large proportion of ferritestrapped with the concentrated magnetite.

In the process according to the invention, by adding an anionicsurfactant (preferably sodium metaphosphate), the positive charge of theferrites is neutralized and can be inverted to attain negative chargeswith an intensity of −40 to −160 mV, and lower. The addition ofsurfactant increases the surface charges of the fine ferrites, decreasesthe cohesion or the attraction between the ferrites and magnetites,causes a stronger repulsion between the particles of ferrites andmaintains these ferrites in suspension. The coarse magnetic fraction,which has a very small specific surface, is not greatly affected by theaddition of surfactant. The granulometry and the mass of the magnetitesenable the decantation of the magnetite with the ferrite in suspension.This procedure substantially improves the results of the magneticseparation and the screening. The condition at the isoelectric point ispreferable in order to optimize the magnetic separation and thescreening (see next section), while controlling the concentration oflead in the solid.

Evaluation of the Results

Magnetic separation in aqueous medium was performed with a drum forwhich a magnetic field was generated by an electromagnet with a maximumpower of about 1200 gauss. The slurry (16), which had a concentration ofsolids of 16% and a mass concentration of surfactant varying from 0.1%to 1.3%, was used in the separation. Magnetic separators are well knownand do not need further description. The slurry (16) was fed with a flowrate of 1 l/min. To unstick the magnetic fraction from the drum, anadditional flow of water (22) of 1.4 l/min was added, totaling 150liters of liquid (to recycle) with a concentration of 3% solids to berecuperated by decantation (60) and screening (30).

The maximum fraction of magnetite recuperated in the pulp varied fromone company to another according to its production. However, the maximumfraction recuperated was on the order of 15 to 20% for the producerusing a pre-reduced hematite mineral and between 8 to 10% for theproducer using scrap iron only.

The quality of separation was qualitatively evaluated under themicroscope by observing the colour, which distinguishes magnetite fromcoal. Colour is also used to evaluate the quality of magneticseparation. Table 7 compares the three components of colour, accordingto the HunterLab color scale, for the raw dust for separation, and forthe separated and screened fractions of ferrites and magnetites. Theparameter “L” of 0,00 corresponds to a black standard used to calibratethe apparatus whereas the value 100,00 is associated with the whitestandard. The parameter “L” indicated a paler shade for the fractionsobtained without the addition of surface active, and which, inconsequence, only contained a concentration of magnetite still coatedwith brown ferrites. On the contrary, with the addition of surfaceactive, the magnetic fraction was of a blacker shade according to theoptical apparatus and also according to the naked eye. TABLE 7 COLORCOMPARISON FOR THE EAF DUST, FERRITE AND MAGNETITE, SEPARATED BYMAGNETIC SEPARATION (MSP) Color Samples L a b Raw Dust 29.09 2.52 8.60from mill 2 Sample before MS 28.63 2.05 8.01 with 0.4% NaMP Sample after28.30 1.75 7.67 20 μm Screening Sample of Raw Magnetite <38 μm 25.73−0.23 3.08

The efficiency of the magnetic separation is supported by the massvalues of the quantity of ferrite trapped by the magnetite. The weightsof the fractions indicate that without the addition of surfactant, theferrite trapped by the magnetite reached a maximum. On the other hand,with the addition of a surfactant, the quantity of ferrite decreased(Table 8). The adsorption of the surfactant occurred preferentially onthe fine fraction of the solid and thus in this case, on the ferrites.The magnetites, being rougher, experienced a change in charge that isless significant and thus there less of an effect on the mobility ofthis phase. TABLE 8 QUANTITY OF FERRITES TRAPPED BY MAGNETITE AS AFUNCTION OF SURFACE ACTIVE MS <6 μm Average weight Difference with v.without Tank Base NaMP g. g. NaMP in g. ou % B99# 143 MP-060/ISP without1933 1961 B99# 144 without 1990 142 g. or 7, 2% B99# 145 with 1849 1819B99# 146 with 1790 S99# 71 MP-060-070- without 712 120/STE 45 g. or 6,3% B99# 68 MP-060-070- with 667 120/STE

Another indication of the efficiency of the separation is provided bythe results of the tests of recuperation of magnetite obtained from therough fraction ≧20 μm after screening the non magnetic fraction. Thisferrite fraction comprised rough contaminants (i.e. coal) and magnetite,with a smaller amount of fine silica and carbonates or calcium phases.The magnetite was not separated in the first magnetic separation as itwas coated with silica and phases of calcium. The trapped quantityvaried with the quantity and concentration of the added surfactant. Fora separation without surface active, 197 g of rough magnetite wasrecovered. The same fraction after having added the surface activeresulted in a recuperation of 221 g, or 11% more magnetite recovered.This result is explained by the fact that the surface active is moreefficient in dispersing fine particles, and thus the finer contaminantsfrom the larger spheroids of magnetite; coal does not influence theseparation.

For the process according to the invention, it is preferable to use thesurfactant according to a specific dosage in order to produce twofractions (24 and 26) that are adequate for realizing products suitablefor commercial applications, as will be explained in more detailsfurther below.

Screening (30 or 32)

Screening of the ferrite fraction (24) or the magnetic fraction (26) isessential to produce ferrite pigments or magnetic pigments having acommercial value, because it allows the physical separation of largeragglomerates and certain contaminants accompanying the ferrites andmagnetites. All particles or agglomerated substances of more than 20 μmwith or without magnetic susceptibility, can be separated. Coal and evenpartially fused scrap metal fragments are separated by screening.

In addition to improving the separation of the ferrites and magnetitesin the first treatment and the magnetic separation, the addition ofsurfactant prevents the clogging of the screens and enables screeningwith openings of 20 to 6 μm. The clogging is caused by portlandite, acalcium hydroxide Ca(OH)₂, which is produced from lime in the raw EAFdust. Portlandite in solution and in suspension is deposited on thewalls of containers and, in particular, on the mesh of the screens, thussealing the latter. By using an appropriate surface active (sodiummetaphosphate), the calcium in solution is precipitated in the form ofcalcium phosphate. This precipitation is associated with the decrease inconductivity observed during the addition of surfactant and thisdecrease continues after reaching the isoelectric point, attaining, incertain cases, a minimum of conductivity (FIG. 9).

The screening tests demonstrated that the more the surface activeconcentration is increased, the more the solution approaches a minimumof conductivity and the less clogging of the screens is observed. Also,the inner walls of the tanks, screens and other equipment can be easilycleaned by simply rinsing with water.

If no addition of a surfactant is used, the portlandite which adheres tosurfaces and screen mesh, must be cleaned with an acidic aqueoussolution. The concentration of surfactant giving the minimum ofconductivity is not preferred because such a high concentration ofsodium metaphosphate interferes with the leaching of lead in the pulp.

The addition of surface active to give the isoelectric point wassufficient to double the slurry flow rate into the screens from 4 l/minto 7 or 8 l/min and thus increase the capacity of filtration. Theaddition of surface active decreased the number of required cleaningsfor a tank of 10 kg using a screen of 20 μm by a factor of three.

In addition to the slurry, a flow of water for screening (32) is used tofacilitate the screening.

In the first to third variants, the rough screened fraction (36) issuedfrom the first screening (30) was subjected to a magnetite separation(210) used to separate the magnetite fraction (212) that remained in theferrite fraction (24). The magnetite fraction (212) was sent to themagnetite production line, as shown in FIG. 5.

Wet Grinding or Grinding by Attrition (50)

This wet grinding can be accomplished with silica sand, zirconium ballsor other materials with a spherical morphology and sufficient hardnessto resist abrasion. The results provided were obtained with thezirconium beads with a range of granulometry of 0.4 to 0.6 mm in ahorizontal grinder, with horizontal type disks. The grinding conditionsand results are presented in Table 9. TABLE 9 WET GRINDING CONDITIONSAND RESULTS FOR FERRITES 1. Wet Grinding Conditions for Ferrites FeedPulp Disk flow rate conc.* speed Amperage Media Load Condition 0.2 l/min350 g/l 27k 3.8 Zr/Sr 80% #1-2 FPM 0.4-0.6 mm Condition 0.5 l/min 350g/l 27k 3.7 Zr/Sr 80% #1-4 FPM 0.4-0.6 mm 2. Wet Grinding Results forFerrites Surface area Volume Viscocity micron (APS) micron (APS) cpsCondition #1-2 0.276 0.432 810 Condition #1-4 0.252 0.517 470 pulpbefore grinding 0.415 6.307 124*The pulp used in the optimization tests is a B99

The goal of this grinding is to break the large aggregates of more than5 to 20 μm in order to give the ferrite pigment particles a restrictedrange of granulometry, more specifically, a bell curve distribution witha median around 0.3 μm. The granulometric distribution after wetgrinding assures that the fraction of rough aggregates of the dust iseliminated and transferred into the range of fine granulometry. Theobtained diameter (in surface) is from 0.25 to 0.28 μm, with a bellcurve distribution desired for the pigments. The results are illustratedin FIGS. 10, 11, 12 and 13. FIGS. 11 and 12 illustrate the granulometricdistributions for slurries after one and two passes in the grinder. Formore aggressive leaching processes, as for the second grade ferritepigment, the slurry does not require grinding, the granulometric medianbeing already close to or just under 0.8 μm. Normally, the first gradeferrite pigment requires grinding in order to obtain an adequatedispersion. Also, some dusts may contain enough aggregates around 20 μmas to require the use of a wet grinder. For cement additives, wetgrinding is necessary, because it decreases the granulometry, increasesthe surface contact between the particles, and generates new surfacesfor a more efficient leaching at the second treatment (40).

The ferrite pigment particles, even after grinding, are still aggregatesof fine nanometric particles. FIG. 14 (AFM microscope) confirms thisstate of agglomeration and the fine size of the constituent beads orfragments.

Second Treatment (40) of the Ferrite Fraction

The goal of the second treatment (40) is to leach the heavy metals stillin the slurry, to eliminate the less stable ferrites and give certainrequired surface characteristics to the pigments (sign and zetapotential intensity), in order to improve the pigment compatibility inpaints, plastics and concrete.

The chemical composition of the pigmentary spinels resulting from thesecond treatment (40) is represented by the chemical compositions givenin Table 10. These pigments represent various slightly differingferrites or spinels rich in iron, zinc, magnesium and manganese andcontain the elements Al, Si, Pb, Ni, Cr etc, as minority components. Allminority components are stabilized in the structure of the spinels andthe lead adheres to the leachate criteria of the TCLP and to the normsand expectations used by paint manufacturers of which the most stringentimposes a maximum concentration of 500 ppm of lead in paint. TABLE 10VARIATION OF CHEMICAL COMPOSITION OF THE PIGMENTS FERRITE IN FUNCTION OFpH SAMPLES Fm# Fm# 1226 1217 Fm# 1314 Fm# 1491 Elements Code Units pH3.0 pH 2.0 pH 1.5 pH 0.5 AL ICP90 ppm 4500 4000 4100 3400 Ba ICP90 ppm70 51 45 n.d. C CHM118 ppm 3700 3700 n.d. n.d. Ca ICP90 ppm 8800 59007200 6700 Cd ICP90 ppm 107 101 112 90 Co ICP90 ppm 11 <10 47 n.d. CrICP90 ppm 1580 1710 1920 1745 Cu ICP90 ppm 2560 2510 2600 2645 Fe ICP95ppm >30 >30 >30 528100 K ICP90 ppm 600 400 500 n.d. Mg ICP90 ppm 2460026800 28700 30100 Mn ICP90 ppm 24450 27090 28260 25900 Mo ICP90 ppm <10<10 11 n.d. Na ICP95 ppm 2700 2700 n.d. n.d. Ni ICP90 ppm 187 187 219n.d. P ICP90 ppm 11400 400 600 n.d. Pb ICP90 ppm 10870 3780 3030 1685 SCHM12 ppm 100 100 n.d. n.d. Si ICP95 ppm 12900 8800 n.d. n.d. Ti ICP90ppm 600 600 600 n.d. V ICP90 ppm 108 110 102 n.d. Zn ICP90 ppm 8914091470 117300 100600

As an example, the effect of the second treatment is illustrated inTable 10 by the variation of lead for the different third grade pigmentleached at different pHs with nitric acid. The most important variationsare the lead concentrations and the zeta for the different pigments. Thesign of the relative charge represented by the zeta potential in aqueousmedium is particularly important, the latter changing from +40mV for thefirst grade to −9 to 11 mV for the leached pigment at a pH of 1.5 to0.5. This parameter is important for the behavior of the pigment andalso influences the pigmentary properties and the coating mechanism, oreven the type of coating it can accept, if required.

Conditions for the Second Treatment (40)

A pulp of 8 to 10% solids in 55 liters of water was acidified withnitric acid 6 N to the desired pH by continuous addition of acid for aperiod of 30 min. The pH was maintained for 60 min. by sporadicallyadding the acid while agitating the pulp. Decantation was preferred andthe surpernatant liquid was removed.

In the first variant, simply water is used as the leaching agent. In thesecond variant, sulfuric acid (42) is used, and in the third variant,nitric acid (43) is used as the leaching agent.

Production of Ferrite/Magnetite Pigments (Seventh Variant)

Referring to FIG. 6, and in accordance with the seventh variant used toproduce ferrite/magnetite pigment, the slurry (16) from the firsttreatment (10) was not subjected to magnetite separation. The slurry wasrather subjected to a screening at the 60 μm or less. The finerfraction, hereinafter referred as to the refined slurry (33) wassubjected to the second leaching treatment (40) with nitric acid (43) ata pH of about 3, to obtain a leached slurry (48) with no or a controlledamount of ZnO which retards the setting of concrete. The leached slurry(48) was separated into a solid fraction (74) containing a mixture offerrite and magnetite pigments and a liquid fraction (72) containingconstituents soluble in nitric acid. The solid fraction (74) was thendried (90) to obtain dry pigments containing a mixture of ferrite andmagnetite.

Specific Characteristics of the Ferrite Pigment of the Third Grade

The pigmentary properties for the ferrite pigments of the third gradeare shown in Table 11 along with the commercial pigments recognized asferrites. These commercial ferrites are obtained by mixing oxidesaccording to a company-specific formulation and then calcining at hightemperature. The table shows different important quantitative pigmentaryproperties such as:

-   -   pH;    -   humidity;    -   “long oil” absorption    -   dry colour of pigment;    -   paint colour;    -   gloss;    -   dispersion on the Hegman gage;

resin incorporation time TABLE 11 PIGMENTARY PROPERTIES OF FERRITEPIGMENTS FM # 1000 1017 1323 1275 1224 Grade B-HS4 B-HS (second B-PE(first Brun 10 130BM (third grade) grade) grade) Note: pH 1.5 0.3% NaMPwith Wet MS with Grinding surfactant Shepherd Bayer (WG) pH 1.0 WGnormal Dispersion: STM1/020 STM1/020 STM1/020 STM1/020 STM1/020 1¼ 1¾ 45sec. 40 sec. 6½ — 6-5½ LN0 — — 6½-7 N Full Nibs 6¾ N 5½ 6¼ N 5 5¾ N 4 5¼Finished Paint: 10% 20% 10% 20% 10% 20% 10% 20% 10% 20% 15.63 22.4713.74 14.71 13.20 13.74 15.17 15.47 8.76 25.04 3.29 3.00 3.30 3.21 3.373.20 7.22 12.21 5.53 5.16 5.47 5.25 6.43 6.25 80.6 47.3 75.3 73.4 69.463.7 82.0 86.2 93.3 81.1 23.2 22.2 99.0 98.2 100.0 100.0 100.0 100.01.89 2.15 1.74 1.89 2.30 2.39 1.91 2.19 1.73 1.89 2.29 2.39 PigmentaryProperties: pH 8.9 7.4 6.3 5.7 11.7 Humidity 0.17 0.26 0.75 0.45 1.47Oil 9.2 18.5 12.0 12.0 20.3 absorption Soluble 0.12 0.14 0.23 0.41 salts325 mesh 0.022 0.853 0.039 0.059 0.098 residu Bulk 0.83 0.43 Density1.20 0.64 Dry color: MP 27.56 31.80 27.07 24.61 29.70 Humidity 9.0425.41 2.65 2.83 2.20 7.95 12.13 8.03 7.40 8.05

Another advantage of ferrite pigment of the third grade is its colourstability at temperatures exceeding 300° C. Table 12 shows the colourparameters for a ferrite before and after heating to 300° C. TABLE 12COLOR PARAMETER CORRECTION FOR HS4 FERRITE PIGMENT (THIRD GRADE) BEFOREAND AFTER HEATING TO 300° C. FOR 60 MIN. Temp. Time Color FM# Grade ° C.min. L a 1257 B99-HS4 (third grade) 300 0 26.37 1.88 pH 1.50 60 26.822.53

Salt mist tests for the pigments for which the properties were presentedin the preceding section, are given in Table 13 for exposure times of500, 1000 and 1500 hours, in a chamber designed for acceleratedcorrosion testing. TABLE 13 SALT MIST ACCELERATED CORROSION TESTS FORFERRITES Performance of coatings after 1512 h Swell- DIC* Groove ZoneGroove Zone Corrosion Formation ing Peeling Larg. Surface FormationCoating Systems/ ASTM of blisters ASTM ASTM Max. rust of blistersProducts D1654/10 ASTM D714 D1654 ASTM D1654/10 mm ASTM D610/10 D714/10Remarks FM#1000-2 7.5 7 G, 8M 8 8.5 4 10 0 56.6 visual: 7 B-PE (firstgrade) 1 P FM#1017-3 7.5 4 G, 1M 8.5 8.5 10 10 0 63.4 visual: 6 B-PE(first grade) 6 P FM#1174-4 8 0 G, 3M 8 7.5 6 10 0 59.2 visual: 3 B-PE(first grade) 0 P FM#1267-1 8.5 0 G, 1M 8.5 9 5 10 0 62.2 visual: 2 Zincchromate 1 P FM#1276-4 8.5 0 G, 0M 8.5 9.5 3 10 0 40.2 visual: 1Strontium chromate 1 P FM#1341-3 8.5 1 G, 1M 8.5 8.5 10 10 0 54.4visual: 4 S-HS4 (third grade) 2 P pH 1, 5 FM#1364-1 8 1 G, 5M 8 8 9 10 053.3 visual: 5 S-HS4 (third grade) 1 P pH 0, 5 Acceptable code 7 à 10 7à 10 6 à 10 8 à 10 *DIC: Corrosion induced loss of adherence **ND: Notdeterminable Position 1512 h FM# 1000 Brun 10 of Shepherd 6 FM# 1017 130MB of Bayferrox 7 FM# 1174 B-PE 3 FM# 1267 Basic Zinc Chromate 233 2 FM#1276 Strontium Chromate 177 1 FM# 1341 S-HS4 pH 1.5 (0.3% NaMP ingranules) 4 FM# 1364 S-HS4 pH 0.5 (0.3% NaMP) 5

Specific Characteristics of the Magnetite Pigments

Magnetite production uses the same treatment units with the exception ofan impact grinder and a 6 μm screen. Normally, magnetite does notrequire leaching with acid and its surface characteristics are moreconstant.

Two magnetites undergo wet grinding: (1) the magnetite fraction afterimpact grinding, between 38 and 6 μm and (2) the fraction of ≦6 μm afterthe purification with the surfactants. In both cases, the particles aretoo coarse or large in diameter to be classified as pigments and requireattrition. Zirconium beads of 0.4 to 0.6 or 0.8 mm were used to attain amedian particle size of 0.3 μm. The initial concentration of the pulpwas 350 g/l and the grinding was performed continuously until thedesired granulometry was obtained.

The magnetite requires purification by putting ferrites and othercontaminants such as calcium and silica into suspension. This suspensionis accomplished with the aid of an anionic surfactant such as sodiummetaphosphate or saratan. The required dosage, in order to optimize thesuspension, is obtained after titrating the pulp with the surfactant.

The results for the magnetite pigmentary properties of the presentinvention and the competitors' pigment properties are shown in Table 14.TABLE 14 PIGMENTARY PROPERTIES OF MAGNETITE PIGMENTS FM # 1335 1336 12391280 Grade B-Mag-PE (M1) B-Mag-HS4 (M2) Shepherd Black Bayferrox 303TNote: WG: SM 1.5/2 h WG: SM 1.5/2 h 376 Magnetite Dispersion: Formulaused: SPTM1/020 SPTM1/020 SPTM1/020 SPTM1/020 Incorporation: (min.) 2115 40 sec. 30 sec. Hegman: (N.S.) from 0-8 from 4-8 N 3 N 1 6¾ N 4½ 8Dispersion: Formula used: SPTM1/020 SPTM1/020 SPTM1/020 SPTM1/020Incorporation: (min.) 21 15 40 sec. 30 sec. Hegman: (N.S.) from 0-8 from4-8 N 3 N 1 6¾ N 4½ 8 Oil absorption (%) 23.1 21.3 12.0 14.8 Solublesalts (%) 0.14 0.13 0.15 0.23 325 mesh (%) 0.059 0.024 0.007 0.0194residu Bulk non-compacted 0.38 0.43 0.69 0.94 Density compacted 0.630.71 1.24 1.33 Dry color: L 24.55 23.61 18.41 19.71 a 0.07 −0.10 −1.03−0.99 b 3.86 3.99 1.00 0.21 MP L a b Humidity (%)

The salt mist tests are also represented in this table for themagnetites.

Magnetite has morphologic and magnetic properties that enable it to beused in inks (Toner) of photocopiers.

Although preferred embodiments for carrying out the invention weredescribed in detail above and illustrated in the annexed drawing, theinvention is not limited to these preferred embodiments, and manychanges and modifications can be made by a person skilled in the art,without leaving the framework or the spirit of the invention.

1. A hydrometallurgical process for the treatment of steel mill electric arc furnace (EAF) dust containing agglomerates of small ferrite particles and larger magnetite particles, the ferrite particles coating by adsorption the larger magnetite particles, the dust further containing calcium oxide, zinc oxide and a toxic amount of leachable lead together with minor elements selected from the group consisting of Mg, Cr, Cu, Cd, V, and chlorides, the process comprising the steps of: a) washing the EAF dust in water to dissolve soluble salts, metals and simple oxides contained in the dust, said washing step being performed under agitation and with an alkaline pH; b) decanting the solution of step a) to obtain a supernatant liquid containing the dissolved salts, metals and simple oxides, and a slurry containing ferrites and magnetites, a non toxic amount of leachable lead and a reduced amount of calcium; c) separating the slurry and the supernatant liquid; d) adding to the slurry obtained in step c) an anionic surfactant to disperse the ferrite particles adsorbed on the magnetite particles; and e) treating the slurry from step d) to produce pigments selected from the group consisting of ferrite pigments, magnetite pigments and ferrite/magnetite pigments.
 2. The process according to claim 1, wherein the sequence of steps a) to c) is performed more than one time before adding the anionic surfactant.
 3. The process according to claim 1, wherein the solution obtained in step a) has a positive zeta potential, and the anionic surfactant is added in a concentration sufficient to reduce said zeta potential to or close to the isoelectric point.
 4. The process according to claim 3, wherein said zeta potential is reduced to the isoelectric point.
 5. The process according to claim 1, wherein the anionic surfactant is a phosphate or an equivalent thereof.
 6. The process according to claim 1, wherein the anionic surfactant preferred is sodium metaphosphate.
 7. The process according to claim 1, wherein step e) of treating the slurry comprises the step of: magnetically separating the slurry into a first fraction composed essentially of brownish ferrites and a second fraction composed essentially of black magnetite, the first fraction being less magnetic than the second fraction.
 8. The process according to claim 7, wherein the step of magnetic separation is performed with a magnetic field in the range of 400 to 700 gauss.
 9. The process according to claim 8, wherein the magnetic field is around 550 gauss.
 10. The process according to claim 7, further comprising the step of: processing the first fraction to produce ferrite pigments.
 11. The process according to claim 10, wherein the step of processing the first fraction comprises: removing from the first fraction, particles having a grain size of 20 μm or more, to obtain a refined first fraction leaching said refined first fraction with a solvent, to obtain a leached slurry; separating said leached slurry into a solid fraction containing ferrite pigments and a liquid fraction containing constituents of the first fraction soluble in said solvent; and drying said solid fraction to obtain dry pigments of ferrites.
 12. The process according to claim 11, wherein the solvent is water and the ferrite pigments are ferrite pigments of a first grade.
 13. The process according to claim 11, wherein the solvent is sulphuric acid, the leaching is performed at a pH of 0.5 to 3 and the ferrite pigments are ferrite pigments of a second grade.
 14. The process according to claim 11, wherein the solvent is nitric acid, the leaching is performed at a pH of up to 3, and the ferrite pigments are ferrite pigments of a third grade.
 15. The process according to claim 14, comprising the step of wet grinding the solid fraction to obtain a forth grade of pigments having a finer mean grain size and a lower concentration of lead as compared to the ferrite pigments of the third grade.
 16. The process according to claim 7, comprising the step of: processing the second fraction to produce magnetite pigments.
 17. The process according to claim 16, wherein the step of processing the second fraction comprises the step of: screening at 6 μm to obtain a first finer fraction with particles having a grain size of 6 μm or less; and a coarser fraction with particles having a grain size greater than 6 μm.
 18. The process according to claim 17, comprising the steps of milling said coarser fraction, and removing from the milled coarser fraction the particles having a grain size greater than 40 μm and returning said particles for further milling, and a second finer fraction having particles with a grain size of less than 6 μm, resulting in said coarser fraction containing particles having a grain size between 40 and 6 μm.
 19. The process according to claim 17, wherein it comprises the steps of: wet grinding by attrition the coarser fraction to attain a mean grain size of approximately 0.3 μm; and filtering and drying the grinded coarser fraction, to obtain a magnetite pigment of a first grade.
 20. The process according to claim 17, wherein it comprises the step of: purifying the first and second finer fractions by suspending residual contaminants contained therein with an anionic surfactant, to obtain a purified magnetic fraction; decanting the purified fraction; wet grinding by attrition the purified fraction; and filtering and drying the ground purified fraction, to obtain a magnetite pigment of a second grade.
 21. The process according to claim 1, comprising the steps of: removing from the slurry obtained in step d), particles having a grain size of 60 μm or less, to obtain a refined slurry; leaching the refined slurry with nitric acid at a pH of about 3, to obtain a leached slurry with no or a controlled amount of ZnO which retard the setting of concrete; separating said leached slurry into a solid fraction containing a mixture of ferrite and magnetite pigments and a liquid fraction containing constituents soluble in nitric acid; and drying said solid fraction to obtain dry pigments containing a mixture of ferrite and magnetite. 22-30. (canceled) 